Diverting agent and method of filling fracture in well using the same

ABSTRACT

A problem to be solved of the present invention is to provide a diverting agent which gradually dissolves in water. The present invention relates to a diverting agent containing a polyvinyl alcohol-based resin and a method of filling a fracture using the diverting agent.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Continuation of U.S. patent application Ser. No. 16/782,415,filed Feb. 5, 2020, which is a Continuation of International Pat. Appl.No. PCT/JP2018/030142, filed Aug. 10, 2018, and claims the prioritybenefit of Japanese Pat Appl. Nos.: 2017-155040, filed Aug. 10, 2017;2017-254842, filed Dec. 28, 2017; and 2017-254843, filed Dec. 28, 2017.The entire disclosure of each of the above-identified applications,including the specification, drawings, and claims, is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a diverting agent and a method offilling a fracture in well using the diverting agent. More specifically,the present invention relates to a diverting agent employed at the timeof construction of excavation method using a hydraulic fracturingmethod, and a method of filling a well fracture using the divertingagent.

BACKGROUND ART

For collecting petroleum or other underground resources, a hydraulicfracturing method in which high-pressure water is injected into anunderground shale layer to cause fractures is widely adopted. In thehydraulic fracturing method, at first, a vertical hole (vertical well)with a depth of several thousand meters is excavated vertically by adrill, and then, when the vertical hole reaches the shale layer, ahorizontal hole (horizontal well) with a diameter of ten to several tensof centimeters is excavated horizontally. By filling vertical andhorizontal wells with fluid and pressurizing the fluid, fractures aregenerated from the well. Natural gas, petroleum (shale gas/oil), or thelike in the shale layer flows out from the fracture, and is collected.According to such a method, a resource inflow cross-section of wells canbe increased by generation of fractures and underground resources can beefficiently collected.

In the hydraulic fracturing method described above, prior to generationof fractures due to fluid pressurization, preliminary blasting calledperforation is performed in a horizontal well. By such preliminaryblasting, borings are made from the well to a production layer. Afterthat, by injecting the fracturing fluid into the well, the fluid flowsinto these borings, and a load is applied to the borings. Then,fractures are generated in these borings and grow into fractures in asize suitable for resource collection.

In the hydraulic fracturing method, a part of fractures that has alreadybeen generated is temporarily filled with an additive called a divertingagent in order to grow fractures that have already been generated largeror to generate more fractures. By temporarily filling a part of thefractures with the diverting agent and pressurizing the fracturing fluidfilled in the well in the state, fluid may enter into the otherfractures, so that other fractures can grow large or new fractures canbe generated.

Since the diverting agent is used to temporarily fill the fractures asdescribed above, a diverting agent which can maintain the shape for acertain period of time and disappears by hydrolysis when a natural gas,petroleum, or the like is collected is used. For example, varioustechniques in which a hydrolyzable resin such as polyglycolic acid orpolylactic acid is used as a divergent agent have been proposed.

Patent Literature 1 has proposed a temporary sealing agent for use inwell boring which contains polyglycolic acid having highbiodegradability among a biodegradable aliphatic polyester-based resin.

Patent Literature 2 has proposed a powder containing particles ofpolylactic acid which is a biodegradable resin, the powder in which 50mass % or more of particles do not pass through a sieve having anopening of 500 μm and the particles have an angle of repose of 51 degreeor more.

Furthermore, Patent Literature 3 has proposed hydrolyzable particleshaving a dispersion structure in which fine particles of a polyoxalatehaving a high biodegradability for adjusting the degree of hydrolysis ofthe polylactic acid are distributed in polylactic acid, and having anaverage particle size (D₅₀) of 300 μm to 1000 μm and a roundness inwhich a minor axis/major axis ratio is 0.8 or more.

Moreover, Patent Literature 4 has proposed a polyoxalate particleshaving an average particle diameter (D50) in a range of 300 μm to 1000μm and a roundness in which a minor axis/major axis ratio is 0.8 ormore.

CITATION LIST Patent Literature

-   Patent Literature 1: WO 2015/072317-   Patent. Literature 2: JP-A-2016-56272-   Patent Literature 3: JP-A-2016-147971-   Patent Literature 4: JP-A-2016-147972

SUMMARY OF INVENTION Technical Problem

As described above, the diverting agent is used to temporarily fillfractures formed in the shale layer. Therefore, the shape of thediverting agent needs to be maintained immediately after being added towater. On the other hand, the diverting agent is preferably removed whenpetroleum, natural gas or the like is collected.

That is, a diverting agent that is only partially dissolved in water,can fill fractures for a certain period of time (approximately 30minutes to 1 week) and can be removed by dissolving in water after acertain period of time is required.

However, the temporary sealant described in Patent Literature 1, thepowder described in Patent Literature 2, the hydrolyzable particlesdescribed in Patent Literature 3, and polyoxalate particles described inPatent Literature 4 do not dissolve in water, and the biodegradationrate of these are slow in the low temperature range, so that it takeslong time to remove them.

The present invention has been made in view of the above conventionalcircumstances, and a problem to be solved of the present invention is toprovide a diverting agent that is only partially dissolved in water, canfill fractures for a certain period of time (approximately 30 minutes to1 week) and can be removed by dissolving in water after a certain periodof time.

Solution to Problem

The present inventors have made intensive studies to solve the aboveproblems, as a result, it has been found that the above problems can besolved when a diverting agent contains a polyvinyl alcohol-based resin.The present invention has been accomplished based on this finding.

That is, the present invention relates to the following <1> to <13>.

<1> A diverting agent including: a polyvinyl alcohol-based resin.<2> The diverting agent according to <1>, in which the polyvinylalcohol-based resin has a saponification degree of 90 mol % or more.<3> The diverting agent according to <1> or <2>, in which the polyvinylalcohol-based resin has a dissolution rate of 0.1 mass % to 30 mass %,when 4 g of the polyvinyl alcohol-based resin is charged into 96 g ofwater and is stirred for 180 minutes at 40° C.<4> The diverting agent according to any one of <1> to <3>, in which adegree of crystallinity of the polyvinyl alcohol-based resin is 25% to60%.<5> The diverting agent according to <1>, in which when 1 g of thepolyvinyl alcohol-based resin is immersed in 100 g of water at 40° C., aratio of a dissolution rate after 24 hours with respect to a dissolutionrate after 1 hour of the polyvinyl alcohol-based resin is 2.8 or more.<6> The diverting agent according to <5>, in which when 1 g of thepolyvinyl alcohol-based resin is immersed in 100 g of water at 40° C.,the dissolution rate after 1 hour is less than 30 mass %.<7> The diverting agent according to <5> or <6>, in which when 1 g ofthe polyvinyl alcohol-based resin is immersed in 100 g of water at 40°C., the dissolution rate after 24 hours is 30 mass % or more.<8> The diverting agent according to any one of <5> to <7>, in which thepolyvinyl alcohol-based resin is a modified polyvinyl alcohol-basedresin.<9> The diverting agent according to <8>, in which a modification rateof the modified polyvinyl alcohol-based resin is 0.5 mol % to 10 mol %.<10> The diverting agent according to <1>, in which the polyvinylalcohol-based resin satisfies the following formula (A).

Degree of swelling×elution rate (mass %)≤500  (A)

(In the formula (A), the degree of swelling is a value determinedaccording to the following formula (B), and the elution rate (mass %) isa value determined according to the following formula (C).)

[Equation1] $\begin{matrix}{{{Degree}{of}{swelling}} = \frac{\begin{matrix}{{{Mass}(g){of}{polyvinyl}{alcohol}}‐{{{based}{resin}{after}{swelling}} -}} \\{{{mass}(g){of}{polyvinyl}{alcohol}}‐{{based}{resin}{dried}{after}{swelling}}}\end{matrix}}{{{mass}(g){of}{polyvinyl}{alcohol}}‐{{based}{resin}{dried}{after}{swelling}}}} & (B)\end{matrix}$

(In the formula (B), the mass (g) of the polyvinyl alcohol-based resinafter swelling is a mass (g) of a residual polyvinyl alcohol-based resinobtained by charging 1 g of a polyvinyl alcohol-based resin into 100 gof water, leaving it to stand for 1 day in a thermostatic chamber at 23°C. and collecting by filtration. The mass (g) of the polyvinylalcohol-based resin dried after swelling is a mass (g) after theresidual polyvinyl alcohol-based resin is dried at 140° C. for 3 hours.)

[Equation2] $\begin{matrix}{{{Elution}{rate}\left( {{mass}\%} \right)} = {\left\{ {\begin{matrix}{{1(g)} -} \\{1{(g) \times}}\end{matrix}\frac{{{M{ass}}(g){of}{polyvinyl}{alcohol}}‐{{based}{resin}{dried}{after}{swelling}}}{\frac{\begin{matrix}{{Solid}{fraction}\left( {{mass}\%} \right){of}{polyvinyl}} \\{{alcohol}‐{{based}{resin}}}\end{matrix}}{100}}} \right\} \times 100}} & (C)\end{matrix}$

(In the formula (C), the mass (g) of the polyvinyl alcohol-based resindried after swelling is the same as defined in the formula (B).)

<11> The diverting agent according to <10>, in which the elution rate ofthe polyvinyl alcohol-based resin is 50 mass % or less.<12> The diverting agent according to <10> or <11>, in which the degreeof swelling of the polyvinyl alcohol-based resin is 30 or less.<13> A method of filling a fracture which is a method of temporarilyfilling the fracture generated in a well, including allowing thediverting agent according to any one of <1> to <12> to flow into afracture to be filled with a flow of fluid in the well.

Advantageous Effects of Invention

According to the present invention, a diverting agent that is onlypartially dissolved in water, can fill fractures for a certain period oftime (approximately 30 minutes to 1 week) and can be removed bydissolving in water after a certain period of time can be provided byusing a polyvinyl alcohol-based resin.

Moreover, by using a polyvinyl alcohol-based resin in which the ratio ofthe dissolution rate after 24 hours with respect to the dissolution rateafter 1 hour described later is a certain level or more, a divertingagent of which the shape can be maintained for approximately one hourafter addition to water and the dissolution rate in water afterapproximately 24 hours is increased can be provided.

Furthermore, by using a polyvinyl alcohol-based resin satisfying aspecific formula relating to the degree of swelling and the elutionrate, a diverting agent having good dispersibility in water can beprovided.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the present invention is described in detail below, butthese are preferred and exemplary embodiments, and the present inventionis not limited to these contents.

In the present invention, (meth)allyl means allyl or methallyl,(meth)acryl means acryl or methacryl, and (meth)acrylate means acrylateor methacrylate.

In the present invention, “mass” is synonymous with “weight”.

[Polyvinyl Alcohol-Based Resin]

The diverting agent of the present invention contains a polyvinylalcohol (hereinafter sometimes referred to as PVA)-based resin. Sincethe diverting agent of the present invention contains a PVA-based resin,the agent is water-soluble.

Furthermore, since the diverting agent of the present invention containsa PVA-based resin, the diverting agent is only partially dissolved inwater, can fill fractures for a certain period of time (approximately 30minutes to 1 week) and can be removed by dissolving in water after acertain period of time, even in a relatively low temperature region (forexample, 30° C. to 60° C.).

In collecting petroleum, natural gas or the like, the diverting agent ofthe present invention is used to fill the fracture in the ground.However, the diverting agent of the present invention dissolves inwater, and does not remain in the ground for a long period of time.Therefore, the diving agent of the present invention is very useful andhas a very small environmental load.

The PVA-based resin used in the present invention preferably satisfiesthe following embodiment (i).

Embodiment (i): A PVA-Based Resin Having a Dissolution Rate of 0.1 Mass% to 30 Mass %, when 4 g of the PVA-Based Resin is Charged into 96 g ofWater and is Stirred for 180 Minutes at 40° C.

The PVA-based resin of the embodiment (i) is described in detail below.

The above dissolution rate is more preferably from 1 mass % to 20 mass%, and further preferably from 2 mass % to 10 mass %. When thedissolution rate is too low, the diverting agent tends to remain evenafter its role of filling fractures in the well is completed, and whenthe dissolution rate is too high, the filling period tends to be veryshort.

The method for measuring the dissolution rate is as follows.

(1) 4 g of the PVA-based resin is charged into 96 g of water at 40° C.(2) The mixture is stirred for 180 minutes while the temperature of thewater is kept at 40° C.(3) After stirring for 180 minutes, the undissolved residue is filteredand the concentration of the aqueous solution excluding the residue ismeasured.

The concentration of the aqueous solution is calculated from thefollowing formula by weighing and collecting an appropriate amount of aPVA-based resin aqueous solution, putting the solution in a drier at105° C. and drying for 3 hours, cooling to room temperature, and thenmeasuring the mass of the dried residue.

The concentration (mass %) of the aqueous solution=mass (parts) of dryresidue/mass (parts) of the weighted and collected aqueous solution ofPVA-based resin×100

(4) The amount of the residue is calculated from the aqueous solutionconcentration and the charged amount of the PVA-based resin, and thedissolution rate is obtained.

The degree of crystallinity of the PVA-based resin used in theembodiment (i) is preferably 25% to 60%, more preferably 30% to 55%,still more preferably 35% to 50%, and particularly preferably 40% to50%.

When the degree of crystallinity is too low, the sealing effect tends todecrease, and when the degree of crystallinity is too high, the waterdissolution rate tends to decrease.

The degree of crystallinity is calculated by the following formula bymeasuring the heat of fusion (ΔH) (J/g) of the target PVA-based resin atthe melting point.

Degree of Crystallinity (%)=ΔH/ΔH ₀×100

(ΔH₀ is a heat of fusion of 156.7 (J/g) of the PVA-based resin having asaponification degree of 100 mol %.)

The heat of fusion (ΔH) (J/g) of the PVA-based resin at the meltingpoint is measured using a differential scanning calorimeter (DSC).

First, 5 mg of a sample to be measured is weighed and collected in ameasurement container. The measurement start temperature is −30° C., andthe temperature is raised at a temperature rising rate of 10° C./min andarrived at an attainment temperature of 200° C. to 240° C. (30° C.higher than the melting point). Thereafter, the temperature is loweredto the measurement start temperature at a temperature decreasing rate of10° C./min. The temperature is raised again to a temperature ofapproximately 30° C. higher than the melting point at a temperaturerising rate of 10° C./min. The endothermic peak area of the meltingpoint at the second temperature rise is calculated as the heat of fusionΔH (J/g).

In the calculation of the heat of fusion (ΔH), first, the horizontalaxis of the analysis chart is defined as a temperature axis, a point atwhich the temperature is 5° C. higher than the temperature of the endpoint of the endothermic peak of the DSC curve is defined as point A, apoint at which the temperature is 40° C. lower than the temperature ofthe vertex of the endothermic peak of the DSC curve is set as point B,and a straight line connecting these two points is defined as a baseline. The heat of fusion (ΔH) (J/g) is calculated from the area of theportion surrounded by the base line and the endothermic peak.

The melting point of the PVA-based resin used in the embodiment (i) isusually 140° C. to 250° C., preferably 150° C. to 245° C., and morepreferably 160° C. to 240° C., still more preferably 170° C. to 235° C.,and particularly preferably 180° C. to 230° C.

The melting point is a value measured by a differential scanningcalorimeter (DSC) at a temperature raising/decreasing rate of 10°C./min.

When the PVA-based resin used in the embodiment (i) is designedaccording to the purpose of use, examples of the method of adjusting theperformances thereof include a method of adjusting the saponificationdegree of the PVA-based resin, a method of adjusting the average degreeof polymerization of the PVA-based resin, a method of introducing amodifying group into the PVA-based resin, a method of performing heattreatment on the PVA-based resin, and a method of forming core-shellparticles using the PVA-based resin. In the present application, thecore-shell particles refer to particles composed of a core portion and ashell portion provided on a surface thereof.

The saponification degree (measured according to JIS K 6726) of thePVA-based resin used in the embodiment (i) is usually 70 mol % or more,preferably 90 mol % or more, more preferably 95 mol % or more, and stillmore preferably 98 mol % or more.

When the saponification degree of the PVA-based resin used in theembodiment (i) is high, the dissolution rate after 1 hour of thePVA-based resin can be lower, and temporary filling by the divertingagent of the embodiment (i) in the fracture of the shale layer can beefficiently performed. When the saponification degree of the PVA-basedresin used in the embodiment (i) is high, the dispersibility of thePVA-based resin in water is good.

When the saponification degree of the PVA-based resin used in theembodiment (i) is too low, the water solubility of the PVA-based resinis low, and it tends to take time to be removed.

From the viewpoint of production efficiency, the saponification degreeof the PVA-based resin used in the embodiment (i) is preferably 99.9 mol% or less, more preferably 99.8 mol % or less, and still more preferably99.5 mol % or less.

The average degree of polymerization of the PVA-based resin used in theembodiment (i) is generally from 150 to 4000, preferably from 200 to3000. In the present specification, the average degree of polymerizationof the PVA-based resin is calculated from the viscosity of an aqueoussolution of 4 mass % at 20° C. measured based on JIS K 6726.

The PVA-based resin used in the embodiment (i) may be an unmodified PVAor a modified PVA-based resin.

Examples of the modified PVA-based resin include a copolymer-modifiedPVA-based resin and a post-modified PVA-based resin.

The copolymer-modified PVA-based resin can be produced by copolymerizinga vinyl ester monomer such as vinyl acetate and other unsaturatedmonomers copolymerizability with the vinyl ester monomer and thenperforming saponification.

The post-modified PVA-based resin can be produced by reacting anunmodified PVA with a modified monomer.

Examples of the other unsaturated monomers having copolymerizabilitywith the above-mentioned vinyl ester monomer include olefins such asethylene, propylene, isobutylene, α-octene, α-dodecene, andα-octadecene; unsaturated acids such as acrylic acid, methacrylic acid,crotonic acid, maleic acid, maleic anhydride, itaconic acid, undecylenicacid, or a salt thereof, monoester thereof or dialkyl ester thereof;nitriles such as acrylonitrile and methacrylonitrile; amides such asdiacetone acrylamide, acrylamide, and naethacrylamide; olefin sulfonicacids such as ethylene sulfonic acid, allyL sulfonic acid, methallylsulfonic acid, or a salt thereof; alkyl vinyl ethers; N-acryl amidemethyltrimethylammonium chloride; allyl trimethylammonium chloride;dirnethylallyl vinyl ketone; N-vinyl pyrrolidone; vinyl chloride;vinylidene chloride; polyoxyalkylene (meth)allyl ether such aspolyoxyethylene (meth)allyl ether and polyoxypropylene (meth)allylether; polyoxyalkylene (meth)acrylate such as polyoxyethylene(meth)acrylate and polyoxypropylene (meth)acrylate; polyoxyalkylene(meth)acrylamide such as polyoxyethylene (meth)acrylamide andpolyoxypropylene (meth)acrylamide; polyoxyethylene(1-(meth)acrylamide-1,1-dimethylpropyl) ester; polyoxyalkylene vinylether such as polyoxyethylene vinyl ether and polyoxypropylene vinylether; polyoxyalkylene allylamine such as polyoxyethylene allylamine andpolyoxypropylene allylamine; polyoxyalkylene vinylamine such aspolyoxyethylene vinylamine and polyoxypropylene vinylamine; hydroxygroup-containing α-olefins such as 3-butene-1-ol, 4-penten-1-ol, and5-hexene-1-ol, or acylated products thereof, vinyl ethylene carbonate;2,2-dialkyl-4-vinyl-1,3-dioxolane; glycerin monoallyl ether; vinylcompounds such as 3,4-diacetoxy-1-butene; isopropenyl acetate;substituted vinyl acetates such as 1-methoxy vinyl acetate;1,4-diacetoxy-2-butene; and vinylene carbonate.

Examples of the copolymer-modified PVA-based resin include a PVA-basedresin having a primary hydroxyl group in a side chain thereof. Examplesof the PVA-based resin include a PVA-based resin having a 1,2-diolstructure in a side chain obtained by copolymerization of3,4-diacetoxy-1-butene, vinylethylene carbonate, glycerin monoallylether, or the like; and a PVA-based resin having a hydroxymethyl groupin a side chain obtained by copolymerization of ahydroxymethylvinylidene diacetate such as 1,3-diacetoxymethylenepropane, 1,3-dipropionyloxy-2-methylenepropane, and1,3-dibutyroyloxy methylenepropane or the like.

The post-modified PVA resin can be produced by post-modification of anunmodified PVA. Examples of such post-modification methods includeacetoacetate esterification, acetalization, urethanization,etherification, phosphate esterification, and oxyalkylation of theunmodified PVA.

Among these, a PVA-based resin having a 1,2-diol structure in the sidechain is preferred from the viewpoint that the dissolution rate can beeasily controlled.

The PVA-based resin used in the embodiment (1) is usually used in theform of pellets, powder, or the like. In particular, from the viewpointof filling, it is preferable to use the pellet form.

When powdery PVA-based resin is used as the PVA-based resin in theembodiment (i), the average particle diameter thereof is preferably 10μm to 3000 μm, more preferably 50 to 2000 μm, and still more preferably100 μm to 1000 μm.

When the average particle size of the PVA-based resin is too small, ittends to be difficult to handle due to scattering or the like. When thePVA-based resin is too large, the PVA-based resin is post-modified, andthe reaction tends to be non-uniform when the PVA-based resin ismodified.

In addition, the average particle diameter is a diameter at which avolume distribution for each particle size is measured by laserdiffraction and an integrated value (cumulative distribution) is 50%.

When a pellet-shaped PVA-based resin is used as the PVA-based resin, theaverage degree of polymerization thereof is particularly preferably 200to 1200, and more preferably 300 to 800. When the average degree ofpolymerization is too low, the water dissolution rate is too high, andthe sealing effect tends to decrease. When the average degree ofpolymerization is too high, the water dissolution rate is too low, andthe time until the dissolution and removal tends to be too long.

When a pellet-shaped PVA-based resin is used as the PVA-based resin, aknown method can be used to form such a pellet shape, but it isefficient to extrude into a strand form from an extruder, cut to apredetermined length after cooling, and form columnar pellets.

The columnar pellets have a length of usually 1 mm to 4 mm, preferably 2mm to 3 mm, and a diameter of usually 1 mm to 4 mm, preferably 2 mm to 3mm.

The pellet-shaped PVA-based resin used in the embodiment (i) can beproduced by, for example, a method of melting powder of a PVA-basedresin.

Examples of the melting method include a method of mixing by a mixersuch as a Henschel mixer or a ribbon blender and then melt-kneading witha melt kneader such as a single screw or a twin screw extruder, a roll,a Banbury mixer, a kneader, a Brabender mixer, or the like. Thetemperature at the time of melt-kneading can be appropriately set withina temperature range that is equal to or higher than the melting point ofthe PVA-based resin and does not thermally deteriorate, and ispreferably 100° C. to 250° C., and particularly preferably 160° C. to220° C.

When the PVA-based resin used in the embodiment (i) is produced by aheat treatment, the PVA-based resin obtained by a conventional method issubjected to heat treatment at, for example, from 90° C. to 220° C.,preferably from 90° C. to 180° C., more preferably from 100° C. to 160°C., for from 10 minutes to 600 minutes, preferably from 20 minutes to400 minutes, and more preferably from 30 minutes to 300 minutes toobtain the PVA-based resin. The heat treatment may be performed by aknown method, or may be performed by melt extrusion or the like, besidesa heat treatment using a heat treatment can or the like.

When the PVA-based resin used in the embodiment (i) is produced by amethod to produce core-shell particles, the resin can be obtained from aknown method, for example, a method described in JP-A-2017-048267.

The PVA-based resin of the present invention preferably satisfies thefollowing embodiment (ii).

Embodiment (ii): PVA Based Resin of which a Ratio of a Dissolution Rateafter 24 Hours with Respect to a Dissolution Rate after 1 Hour is 2.8 orMore, when 1 g of the Polyvinyl Alcohol-Based Resin is Immersed in 100 gof Water at 40° C.

That is, the PVA-based resin used in the embodiment (ii) is preferably aPVA-based resin satisfying the following formula (X).

Dissolution rate (mass %) after 24 hours/dissolution rate (mass %) after1 hour≥2.8  (X)

When the ratio of the dissolution rate after 24 hours with respect tothe dissolution rate after 1 hour of the PVA-based resin used in theembodiment (ii) is 2.8 or more, the PVA-based resin tends to maintainthe shape thereof for approximately 1 hour after the addition to water,and increase dissolution rate in water after approximately 24 hours.

Therefore, the diverting agent of the embodiment (ii) containing thePVA-based resin can maintain the shape thereof for a certain period oftime, and can be dissolved in water when petroleum, natural gas or thelike is collected.

From the viewpoint of further increasing the dissolution rate in waterafter approximately 24 hours, the ratio of the dissolution rate after 24hours with respect to the dissolution rate after 1 hour is morepreferably 3.0 or more, still more preferably 3.5 or more, andparticularly preferably 4.0 or more.

In addition, the ratio of the dissolution rate after 24 hours withrespect to the dissolution rate after 1 hour is preferably 20 or less,and more preferably 10 or less, since if the dissolution is too fast,the duration of the sealing effect is too short.

In the formula (X), the dissolution rate after 1 hour is calculated fromthe ratio of mass (mass %) of the PVA-based resin remaining withoutbeing dissolved, after 1 g of the PVA-based resin is immersed in 100 gof water of 40° C. and the mixture is left to stand for 1 hour.Specifically, the dissolution rate (mass %) after 1 hour can becalculated by the following method.

A 140 mL glass container with a lid containing 100 g of water is placedin a thermostatic chamber, and the water temperature is set to 40° C.The long sides of 120 mesh (aperture 125 μm, 1.0 cm×7 cm) made of nylonare folded in half, and both ends are heat-sealed to obtain a mesh bag(5 cm×7 cm).

1 g of the PVA-based resin is put into the obtained mesh bag, theopening is heat-sealed to obtain a mesh bag containing the PVA-basedresin, and then the mass is measured. The mesh bag containing thePVA-based resin is immersed in the glass container. After standing for 1hour, the mesh bag containing the PVA-based resin is taken out from theglass container and then dried at 105° C. for 3 hours. The mass of themesh bag containing the PVA-based resin is measured, the mass of thePVA-based resin remaining in the mesh bag is calculated from the massbefore the immersion, then the dissolution rate (mass %) after 1 hour ofthe PVA-based resin is calculated by the following formula (Y).

In the following formula (Y), the solid fraction (mass %) of thepolyvinyl alcohol-based resin can be calculated by drying the PVA-basedresin at 105° C. for 3 hours and measuring the mass of the PVA-basedresins before and after drying.

[Equation3] $\begin{matrix}{\begin{matrix}{{Dissolution}{rate}} \\{{after}1{hour}} \\{{of}{the}{immersion}} \\\left( {{mass}\%} \right)\end{matrix} = {\left\{ {\begin{matrix}{{1(g)} -} \\{1{(g) \times}}\end{matrix}\frac{\begin{matrix}{{{M{ass}}(g){of}{the}{polyvinyl}{alcohol}}‐} \\{{based}{resin}{remaining}{in}{the}{mesh}{bag}}\end{matrix}}{\frac{\begin{matrix}{{Solid}{fraction}\left( {{mass}\%} \right){of}{polyvinyl}} \\{{alcohol}‐{{based}{resin}}}\end{matrix}}{100}}} \right\} \times 100}} & (Y)\end{matrix}$

Further, in the formula (X), the dissolution rate (mass %) after 24hours can be calculated in the same manner as the calculation of thedissolution rate (mass %) after 1 hour except that standing for 1 houris changed to standing for 24 hours and by calculating the mass of thePVA-based resin remaining in the mesh bag after 24 hours.

The dissolution rate after 1 hour of the PVA-based resin used in theembodiment (ii) is preferably less than 30 mass %, more preferably 25mass % or less, and still more preferably 20 mass % or less.

When the dissolution rate after 1 hour is less than 30 mass %, the shapeof the PVA-based resin can be maintained for a certain period of time,and it can be efficiently performed to temporarily fill the fractures ofthe shale layer by the diverting agent of the embodiment (ii).

The dissolution rate after 1 hour of the PVA-based resin used in theembodiment (ii) is preferably 0.01 mass % or more, more preferably 0.1mass % or more, and still more preferably 1 mass % or more, since whenthe dissolution is too slow, the sealing period is too long.

The dissolution rate after 24 hours of the PVA-based resin used in theembodiment (ii) is preferably 30 mass % or more, more preferably 40 mass% or more, and still more preferably 50 mass % or more.

When the dissolution rate after 24 hour is 30 mass % or more, thediverting agent of the embodiment (ii) can be efficiently removed, whenthe oil, natural gas, or the like is collected.

The dissolution rate after 24 hours of the PVA-based resin used in theembodiment (ii) is preferably 99 mass % or less, more preferably 90 mass% or less, still more preferably 80 mass % or less, and particularlypreferably 70 mass % or less, since when the dissolution is too fast,the sealing period is too short.

Examples of the method of adjusting the ratio of the dissolution rateafter 24 hours with respect to the dissolution rate after 1 hour includea method of adjusting the degree of saponification of the PVA-basedresin, a method of adjusting the average degree of polymerization of thePVA-based resin, a method of introducing a modifying group into thePVA-based resin, a method of performing heat treatment on the PVA-basedresin, and a method of forming core-shell particles using the PVA-basedresin.

When the PVA-based resin satisfying the formula (X) is used, the averagedegree of polymerization (measured based on JIS K 6726) of the PVA-basedresin is preferably 450 or more, more preferably 700 or more, and stillmore preferably 1000 or more.

When the average degree of polymerization of the PVA-based resin is 450or more, the dissolution rate after 1 hour of the PVA-based resin can belower, and it can be efficiently performed to temporarily fill thefractures of the shale layer by the diverting agent of the embodiment(ii).

The average degree of polymerization of the PVA-based resin ispreferably 4000 or less, more preferably 3000 or less, and still morepreferably 2500 or less from the viewpoint of preventing the dissolutionrate after 24 hours from being too low.

The degree of saponification of the PVA-based resin used in theembodiment (ii) (measured according to JIS K 6726) is usually 70 mol %or more, preferably 90 mol % or more, more preferably 95 mol % or more,and still more preferably 98 mol % or more. From the viewpoint ofproduction efficiency, the upper limit of the degree of saponificationis preferably 99.9 mol % or less, more preferably 99.8 mol % or less,and still more preferably 99.5 mol % or less.

When the PVA-based resin used in the embodiment (ii) is produced by aheat treatment, the PVA-based resin can be obtained by subjecting aPVA-based resin obtained by a conventional method to heat treatment at90° C. to 220° C., preferably 90° C. to 180° C., and still morepreferably 100° C. to 160° C., for preferably 10 minutes to 600 minutes,more preferably 20 minutes to 400 minutes, and more preferably 30minutes to 300 minutes. The heat treatment may be performed by a knownmethod, or may be performed by melt extrusion or the like, besides aheat treatment using a heat treatment can or the like.

The PVA-based resin used in the embodiment (ii) may be an unmodified PVAor a modified PVA-based resin, and is preferably a PVA-based resinhaving a 1,2-diol structure in a side chain thereof.

When the PVA-based resin used in the embodiment (ii) is produced by amethod to produce core-shell particles, the resin can be obtained from aknown method, for example, a method described in JP-A-2017-048267.

The PVA-based resin used in the present invention preferably satisfiesthe following embodiment (iii).

Embodiment (iii): A PVA-Based Resin Satisfying the Following Formula (A)

Degree of Swelling×Elution Rate (Mass %)≤500  (A)

In the formula (A), the degree of swelling is a value obtained by thefollowing formula (B).

[Equation4] $\begin{matrix}{{{Degree}{of}{swelling}} = \frac{\begin{matrix}{{{Mass}(g){of}{polyvinyl}{alcohol}}‐{{{based}{resin}{after}{swelling}} -}} \\{{{mass}(g){of}{polyvinyl}{alcohol}}‐{{based}{resin}{dried}{after}{swelling}}}\end{matrix}}{{{mass}(g){of}{polyvinyl}{alcohol}}‐{{based}{resin}{dried}{after}{swelling}}}} & (B)\end{matrix}$

In the formula (B), the mass (g) of the polyvinyl alcohol-based resinafter swelling is a mass (g) of a residual polyvinyl alcohol-based resinobtained by charging 1 g of a polyvinyl alcohol-based resin into 100 gof water, leaving it to stand for 1 day in a thermostatic chamber at 23°C. and collecting by filtration.

In the formula (B), the mass (g) of the polyvinyl alcohol-based resindried after swelling is a mass (g) after the residual polyvinylalcohol-based resin is dried at 140° C. for 3 hours.

In the formula (A), the elution rate (mass %) is a value obtained by thefollowing formula (C).

[Equation5] $\begin{matrix}{{{Elution}{rate}\left( {{mass}\%} \right)} = {\left\{ {\begin{matrix}{{1(g)} -} \\{1{(g) \times}}\end{matrix}\frac{{{M{ass}}(g){of}{polyvinyl}{alcohol}}‐{{based}{resin}{dried}{after}{swelling}}}{\frac{\begin{matrix}{{Solid}{fraction}\left( {{mass}\%} \right){of}{polyvinyl}} \\{{alcohol}‐{{based}{resin}}}\end{matrix}}{100}}} \right\} \times 100}} & (C)\end{matrix}$

In the formula (C), the mass (g) of the polyvinyl alcohol-based resindried after swelling is the same as defined in the formula (B).

In the formula (C), the solid fraction (mass %) of the polyvinylalcohol-based resin can be calculated by drying the PVA-based resin at105° C. for 3 hours and measuring the mass of the PVA-based resinsbefore and after drying.

When the value of the degree of swelling×elution rate (mass %) of thePVA-based resin used in the embodiment (iii) is 500 or less, after thePVA-based resin aqueous solution is allowed to stand for one day,swelling and elution of the PVA-based resin can be suppressed andadhesion of the PVA-based resin to each other can be suppressed, andthus the dispersibility of the diverting agent in the embodiment (iii)in water can be increased.

From the viewpoint of increasing the dispersibility, the value of thedegree of swelling×the elution rate (mass %) is more preferably 0 to500, still more preferably 1 to 480, and particularly preferably 2 to400.

The degree of swelling of the PVA-based resin used in the embodiment(iii) is preferably 30 or less, more preferably 20 or less, and stillmore preferably 10 or less.

When the degree of swelling of the PVA-based resin used in theembodiment (iii) is 30 or less, excessive enlargement of the PVA-basedresin by water can be suppressed, and the load on the pump for feedingwater or the diverting agent of the embodiment (iii) can be reduced. Inaddition, even in the fine fractures in the shale layer, the divertingagent of embodiment (iii) can form filling.

The lower limit of the degree of swelling is 0.

Examples of the method of adjusting the degree of swelling of thePVA-based resin used in the embodiment (iii) include a method ofadjusting the degree of saponification of the PVA-based resin, a methodof adjusting the average degree of polymerization of the PVA-basedresin, a method of introducing a modifying group into the PVA-basedresin, a method of performing heat treatment on the PVA-based resin, anda method of forming core-shell particles using the PVA-based resin.

The elution rate of the PVA-based resin used in the embodiment (iii) ispreferably 50 mass % or less, more preferably 30 mass % or less, andstill more preferably 10 mass % or less.

When the elution rate of the PVA-based resin used in the embodiment(iii) is 50 mass % or less, the shape of the PVA-based resin can bemaintained for a certain period of time, and it can be efficientlyperformed to temporarily fill the fractures of the shale layer by thediverting agent of the embodiment (iii).

The lower limit of the elution rate is 0 mass %.

Examples of the method of adjusting the elution rate of the PVA-basedresin used in the embodiment (iii) include a method of adjusting thedegree of saponification of the PVA-based resin, a method of adjustingthe average degree of polymerization of the PVA-based resin, a method ofintroducing a modifying group into the PVA-based resin, a method ofperforming heat treatment on the PVA-based resin, and a method offorming core-shell particles using the PVA-based resin.

The degree of saponification of the PVA-based resin used in theembodiment (iii) (measured according to JIS K 6726) is usually 70 mol %or more, preferably 90 mol % or more, more preferably 95 mol % or more,and still more preferably 98 mol % or more. From the viewpoint ofproduction efficiency, the upper limit of the degree of saponificationis preferably 99.9 mol % or less, more preferably 99.8 mol % or less,and still more preferably 99.5 mol % or less.

When the PVA-based resin used in the embodiment (iii) is produced by aheat treatment, the PVA-based resin can be obtained by subjecting aPVA-based resin obtained by a conventional method to heat treatment at90° C. to 220° C., preferably from 90° C. to 180° C., and stillpreferably from 100° C. to 160° C., for from 10 minutes to 600 minutes,preferably from 20 minutes to 400 minutes, and more preferably from 30minutes to 300 minutes. The heat treatment may be performed by a knownmethod, or may be performed by melt extrusion or the like, besides aheat treatment using a heat treatment can or the like.

The PVA-based resin used in the embodiment (iii) may be an unmodifiedPVA or a modified PVA-based resin, and is preferably an unmodified PVAor a PVA-based resin having an ethylene group.

When the PVA-based resin used in the embodiment (iii) is produced by amethod to produce core-shell particles, the resin can be obtained from aknown method, for example, a method described in JP-A-2017-048267.

<Production Method of PVA-Based Resin>

The PVA-based resin used in the present invention has a vinyl alcoholstructural unit corresponding to the degree of saponification and avinyl acetate structural unit of an unsaponified portion.

Examples of the PVA-based resin used in the present invention include amodified PVA-based resin obtained by copolymerizing various monomersduring the production of a vinyl ester resin and being saponified, and avariety of post-modified PVA-based resins obtained by introducingvarious functional group into an unmodified PVA by post-modification, inaddition to an unmodified PVA. Such modification can be performed aslong as the water dissolution rate of the PVA-based resin is not lost.In some cases, the modified PVA-based resin may be furtherpost-modified.

Examples of monomer used for copolymerization with vinyl ester monomersin the production of a vinyl ester resin include olefins such asethylene, propylene, isobutylene, α-octene, α-dodecene, andα-octadecene; unsaturated acids such as acrylic acid, methacrylic acid,crotonic acid, maleic acid, maleic anhydride and itaconic acid, a saltthereof, monoester thereof or dialkyl ester thereof, or the like;nitriles such as acrylonitrile and methacrylonitrile; amides such asacrylamide and methacrylamide; olefin sulfonic acid or a salt thereofsuch as ethylene sulfonic acid, allyl sulfonic acid, and methallylsulfonic acid; alkyl vinyl ethers; N-acryl amide methyltrimethylammoniumchloride; allyl trimethylammonium chloride; dimethylallyl vinyl ketone;N-vinyl pyrrolidone; vinyl chloride; vinylidene chloride;polyoxyalkylene (meth) allyl ether such as polyoxyethylene (meth) allylether and polyoxypropylene (meth) allyl ether; polyoxyalkylene (meth)acrylate such as polyoxyethylene (meth) acrylate and polyoxypropylene(meth) acrylate; polyoxyalkylene (meth) acrylamide such aspolyoxyethylene (meth) acrylamide and polyoxypropylene (meth)acrylamide; polyoxyethylene (1-(meth)acrylamide-1,1-dimethylpropyl)ester; polyoxyalkylene vinyl ether such as polyoxyethylene vinyl etherand polyoxypropylene vinyl ether; polyoxyalkylene allylamine such aspolyoxyethylene allylamine and polyoxypropylene allylamine;polyoxyalkylene vinylamine such as polyoxyethylene vinylamine andpolyoxypropylene vinylamine; a hydroxy group-containing α-olefins suchas 3-buten-1-ol, 4-penten-1-ol, and 5-hexen-1-ol, or a derivativethereof such as an acylated product thereof.

In addition, examples thereof include a compound having a diol such as3, 4-dihydroxy-1-butene, 3,4-diacyloxy-1-butene,3-acyloxy-4-hydroxy-1-butene, 4-acyloxy-3-hydroxy-1-butene,3,4-diacyloxy-2-methyl-1-butene, 4,5-dihydroxy-1-pentene,4,5-diacyloxy-1-pentene, 4,5-dihydroxy-3-methyl-1-pentene,4,5-diacyloxy-3-methyl-1-pentene, 5,6-dihydroxy-1-hexene,5,6-diacyloxy-1-hexene, glycerin monoallyl ether,2,3-diacetoxy-1-allyloxypropane, 2-acetoxy-1-allyloxy-3-hydroxypropane,3-acetoxy-1-allyloxy-2-hydroxypropane, glycerin monovinyl ether,glycerin monoisopropenyl ether, vinylethylene carbonate,2,2-dimethyl-4-vinyl-1,3-dioxolane, or the like.

Examples of the post-modified PVA-based resin obtained by introducingfunctional group by post-reaction include those having an acetoacetylgroup by reaction with diketene, those having a polyalkylene oxide groupby reaction with ethylene oxide, those having a hydroxyalkyl group byreaction with an epoxy compound or the like, or those obtained byreacting an aldehyde compound having various functional groups with aPVA-based resin, or the like.

The PVA-based resin used in the present invention is preferably amodified PVA-based resin, and more preferably a modified PVA-based resinhas a hydrophilic modifying group, from the viewpoint of furtherincreasing the ratio of the dissolution rate after 24 hours with respectto the dissolution rate after 1 hour.

When the PVA-based resin used in the present invention is a modifiedPVA-based resin, the modification rate in the modified PVA resin, thatis, the content of a structural unit derived from various monomers inthe copolymer, or the functional group introduced by the post reactionis preferably 0.5 mol % to 10 mol %, more preferably 0.7 mol % to 8 mol%, and still more preferably 1.0 mol % to 5 mol %, from the viewpoint ofincreasing the ratio of the dissolution rate after 24 hours with respectto the dissolution rate after 1 hour.

The modification rate of the PVA-based resin used in the presentinvention can be determined from the ¹H-NMR spectrum (solvent, DMSO-d₆;internal standard, tetramethylsilane) of the PVA-based resin having adegree of saponification of 100 mol %. Specifically, the modificationrate can be calculated from a peak area derived from a hydroxyl proton,a methine proton, and a methylene proton in the modifying group, amethylene proton in the main chain, a proton of a hydroxyl group linkedto the main chain, or the like.

Examples of the hydrophilic modifying group include an oxyalkylenegroup, a hydroxyl group-containing alkyl group, an amino group, an aminogroup-containing alkyl group, a thiol group, and a thiolgroup-containing alkyl group. Among these, an oxyalkylene group and ahydroxyl group-containing alkyl group are preferably used, since theeffects of the present invention can be remarkably obtained.

Examples of the oxyalkylene group include an oxyethylene group and anoxyethylene-oxypropylene copolymer group. In the case of anoxyethylene-oxypropylene copolymer, since the hydrophilicity tends todecrease as the oxypropylene component increases, an oxyethylene groupis preferably used.

Many examples of the hydroxyl group-containing alkyl group are givendepending on the number of carbon atoms of the alkyl group, the numberof hydroxyl groups, the valence of hydroxyl groups, the bonding mode, orthe like. The number of carbon atoms of the alkyl group is usually 1 to5, particularly preferably 2 to 3. The number of hydroxyl groups isusually 1 to 4, more preferably 1 to 3, and the valence number thereofis preferably a primary hydroxyl group. In particular, among thesehydroxyl group-containing alkyl groups, a 1,2-dial group in which aprimary hydroxyl group and a secondary hydroxyl group are bonded toadjacent carbon atoms is preferred.

Since the terminal of the oxyethylene group is usually a hydroxyl group,the oxyethylene group is contained in the hydroxyl group-containingfunctional group.

Further, the PVA-based resin used in the present invention may be amixture with other different PVA-based resin, and examples of the otherPVA-based resins include those having different contents of themodifying group, those having different degrees of saponification, thosehaving different average degrees of polymerization, those havingdifferent other copolymerization components, and those having nomodifying group. When a mixture is used, the average value of thedegrees of saponification, the degree of polymerization, and themodification rate is preferably within the above-described range.

Further, as the PVA-based resin used in the present invention, thoseobtained by copolymerizing various unsaturated monomers can be used aslong as the objects of the present invention are not inhibited. Theintroduction amount of such an unsaturated monomer is generally lessthan 10 mol %. When the introduction amount is too large, it is likelythat the hydrophilicity is impaired.

Hereinafter, in the PVA-based resin used in the present invention, thePVA-based resin having an oxyethylene group preferably used as amodifying group in the side chain (hereinafter sometimes referred to asan oxyethylene group-containing PVA-based resin) will be described indetail.

The oxyethylene group is represented by the following general formula(1).

In the general formula (1), n represents a positive integer, and n isusually 5 to 50, preferably 8 to 20. n is an average value of the numberof oxyethylene groups contained in the PVA-based resin.

R¹ and R² each represent a hydrogen atom or an alkyl group having 1 to 3carbon atoms. Examples of the alkyl group having 1 to 3 carbon atomsinclude a methyl group, an ethyl group, an n-propyl group, and anisopropyl group. The alkyl group may have a substituent such as ahalogen group, a hydroxyl group, an ester group, a carboxylic acid groupor a sulfonic acid group as necessary.

An oxyalkylene group other than the oxyethylene group, for example, anoxypropylene group may be copolymerized in a small amount as long as thehydrophilicity is not impaired.

The oxyethylene group-containing PVA-based resin can be obtained bysaponifying a modified polyvinyl ester resin obtained by copolymerizinga vinyl ester monomer and an unsaturated monomer having an oxyethylenegroup.

Examples of the unsaturated monomer having an oxyethylene group includevarious monomers, and typical examples thereof are as follows.

((Meth)Acrylic Ester Type) (Meth)acrylic ester type is represented bythe following formula (2), and specific examples thereof includepolyoxyethylene (meth)acrylate.

In the formula (2), Y represents a hydrogen atom or a methyl group, andn, R¹ and R² are the same as defined above.

((Meth)Acrylamide Type)

(Meth)acrylamide type is represented by the following formula (3), andspecific examples thereof include polyoxyethylene (meth)acrylamide.

In the formula (3), Y, n, R¹ and R² are the same as defined above.

((Meth)allyl Ether Type)

(Meth)allyl Ether Type is represented by the following formula (4), andspecific examples thereof include polyoxyethylene (meth)allyl ether orthe like.

In the formula (4), Y, n, R¹ and R² are the same as defined above.

(Vinyl Ether Type)

Vinyl ether type is represented by the following formula (5), andspecific examples thereof include polyoxyethylene vinyl ether,polyoxypropylene vinyl ether, or the like.

In the formula (5), n, R¹ and R² are the same as defined above.

Among monomers containing these oxyethylene groups, those of the(meth)allyl ether type represented by the formula (4) are preferablyused from the viewpoints of ease of the copolymerization reaction,stability in the saponification step and the like.

Examples of the vinyl ester monomer copolymerizable with the abovemonomers include vinyl formate, vinyl acetate, vinyl trifluoroacetate,vinyl propionate, vinyl butyrate, vinyl caprate, vinyl laurate, vinylversatate, vinyl palmitate, vinyl stearate, vinyl pivalate, or the likeare used alone or in combination. Industrially, vinyl acetate ispreferred.

The copolymerization is not particularly limited, and a knownpolymerization method is used.

Hereinafter, in the PVA-based resin used in the present invention, thePVA-based resin having a 1,2-diol group preferably used as a modifyinggroup in the side chain (hereinafter sometimes referred to as a 1,2-diolgroup-containing PVA-based resin) will be described in detail.

The 1,2-diol group-containing PVA-based resin is a PVA-based resinhaving a 1,2-diol structural unit represented by the following generalformula (6).

In the general formula (6), R⁵ to R¹⁰ each independently represent ahydrogen atom or an alkyl group having 1 to 5 carbon atoms, and Xrepresents a single bond or a bond chain.

Examples of the alkyl group having 1 to 5 carbon atoms include a methylgroup, an ethyl group, an n-propyl group, and an isopropyl group, and ann-butyl group, an isobutyl group, a tert-butyl group, and a pentylgroup. The alkyl group may have a substituent such as a halogen group, ahydroxyl group, an ester group, a carboxylic acid group, or a sulfonicacid group as necessary.

All of R⁵ to 10 are preferably hydrogen atoms, but any of R⁵ to R¹⁰ maybe an alkyl group having 1 to 5 carbon atoms as long as the resinproperties are not remarkably impaired.

X is preferably a single bond in terms of thermal stability andstability under high temperature and acidic conditions, but may be abond chain as long as the effect of the present invention is notimpaired.

The bond chain is not particularly limited, and, for example, a linearor branched alkylene group having 1 to 4 carbon atoms, a linear orbranched alkenylene group having 1 to 4 carbon atoms, a linear orbranched alkynylene group having 1 to 4 carbon atoms, and hydrocarbonssuch as phenylene group and naphthylene group (these hydrocarbons may besubstituted with halogens such as fluorine, chlorine, bromine, etc.),—O—, —(CH₂O)_(m)—, —(OCH₂)_(m)—, —(CH₂O)_(m)CH₂—, —CO—, —COCO—,—CO(CH₂)_(m)CO—, —CO(C₆H₄)CO—, —S—, —CS—, —SO—, —SO₂—, —NR—, —CONR—,—NRCO—, —CSNR—, —NRCS—, —NRNR—, —HPO₄—, —Si(OR)₂—, —OSi(OR)₂—,—OSi(OR)₂O—, —Ti(OR)₂—, —OTi(OR)₂—, —OTi(OR)₂O—, —Al(OR)—, —OAl(OR)—,—OAl(OR)O—, or the like. R each independently represents a hydrogen atomor an optional substituent, and a hydrogen atom or an alkyl group(particularly an alkyl group having 1 to 4 carbon atoms) is preferred.

In addition, m is a natural number, preferably 1 to 10, and particularlypreferably 1 to 5.

Among these, an alkylene group having 1 to 4 carbon atoms, particularlya methylene group, or —CH₂OCH₂— is preferred in terms of viscositystability and heat resistance during production.

The 1,2-diol group-containing PVA-based resin can be produced by a knownproduction method. For example, the PVA-based resin can be produced bymethods described in JP-A-2002-284818, JP-A-2004-285143, andJP-A-2006-95825.

That is, the PVA-based resin can be produced by (i) a method ofsaponifying a copolymer of a vinyl ester monomer and a compoundrepresented by the following general formula (7), (ii) a method ofsaponifying and decarboxylation of a copolymer of a vinyl ester monomerand a vinylethylene carbonate represented by the following generalformula (8), and (iii) a method of saponifying and deketalizing acopolymer of a vinyl ester monomer and a2,2-dialkyl-4-vinyl-1,3-dioxolane represented by the following generalformula (9), or the like.

In the general formula (7), each of R⁵ to R¹⁰ and X is the same asdefined in the general formula (6). R¹¹ and R¹² each independentlyrepresent a hydrogen atom or R¹³—CO— (in the formula, R¹³ represents analkyl group of 1 to 5 carbon atoms.)

In the general formula (8), each of R⁵ to R¹⁰ and X is the same asdefined in the general formula (6).

In the general formula (9), each of R⁵ to R¹⁰ and X is the same asdefined in the general formula (6). R¹⁴ and R¹⁵ each independentlyrepresent a hydrogen atom or an alkyl group of 1 to 5 carbon atoms.

Specific examples of the alkyl group having 1 to 5 carbon atoms of R¹³to R¹⁵ are the same as those in the general formula (6).

Among the above methods, the method (i) is preferred in thatcopolymerization reactivity and industrial handling are excellent. Inparticular, the compound represented by the general formula (7) ispreferable to use a 3,4-diacyloxy-1-butene in which R⁵ to R¹⁰ arehydrogen atoms, X is a single bond, R¹¹ and R¹² are R¹³—CO—, and R¹³ isan alkyl group having 1 to 5 carbon atoms. Among these,3,4-diacetoxy-1-butene in which R¹³ is a methyl group is particularlypreferably used.

In addition, it is possible to obtain the PVA-based resin of whichapproximately 1.6 mol % to 3.5 mol % of the 1,2-diol structure isintroduced in the main chain, by setting the polymerization temperatureto 100° C. or more.

[Diverting Agent]

The diverting agent of the present invention contains the PVA-basedresin described above. The content of the PVA-based resin is usually 50mass % to 100 mass %, preferably 80 mass % to 100 mass %, andparticularly preferably 90 mass % to 100 mass % with respect to theentire diverting agent. When the content is too small, the effects ofthe present invention tend to be difficult to achieve.

In addition to the PVA-based resin, additives such as sand, iron,ceramic, and other biodegradable resins can be blended in the divertingagent of the present invention.

The amount of the additive blended is usually 50 mass % or less,preferably 20 mass % or less, and still more preferably 10 mass % orless with respect to the entire diverting agent.

The shape of the diverting agent of the present invention is usually acolumnar shape (pellet), a spherical shape, a powder shape, or the like,and is preferably a columnar shape or a powder shape in view ofimproving or producing a sealing effect, and is preferably a mixturethereof when used.

In the case of the columnar shape (pellet), the diameter is usually from0.5 mm to 4.0 mm, preferably from 1.0 trim to 3.0 mm, particularlypreferably from 1.85 mm to 2.25 mm, and the thick is usually from 0.5 mmto 4.0 mm, preferably from 1.0 mm to 3.0 mm, particularly preferablyfrom 1.85 mm to 2.25 mm.

In the case of a powder form, the average particle diameter ispreferably 10 μm to 3000 μm, more preferably 50 μm to 2000 μm, and stillmore preferably 100 μm to 1000 μm. The average particle diameter is adiameter in which a volume distribution for each particle size ismeasured by laser diffraction and an integrated value (cumulativedistribution) is 50%.

When the diameter, the thickness, and the average particle diameter aretoo large, water dissolution rate tends to decrease, and when thediameter, the thickness, and the average particle diameter are toosmall, the sealing effect tends to decrease.

When oil, gas, or the like is excavated in a hydraulic fracturingmethod, the diverting agent of the present invention enters into aformed fracture or fissure, temporarily fill the fracture or fissure,and then can form a new fracture or fissure. As a method for filling afracture or fissure, the diverting agent of the present invention isallowed to flow into a fracture to be filled with a flow of fluid in thewell.

Further, since the diverting agent of the present invention iswater-soluble and biodegradable, the diverting agent is rapidlydissolved in water after use, and is then biodegraded. Therefore,environmental load is small, and the diverting agent is very useful.

It should be noted that some or all of the elements and features in theabove embodiments may be appropriately combined with other embodiments.

EXAMPLE

Hereinafter, the present invention will be described in detail withreference to Examples, but the present invention is not limited thereto.

In Examples, “parts” and “%” mean mass basis unless otherwise specified.

Test Example 1

[No. 1-1]

[Production of PVA 1-1]

10 parts of vinyl acetate (10% of the total was used for initial charge)and 45 parts of methanol were added to a reaction can equipped with areflux condenser, a dropping device, and a stirrer, the temperature wasraised under a nitrogen stream while stirring. After reaching theboiling point, 0.050 parts of acetyl peroxide were charged, andpolymerization was started.

After 0.28 hours from the start of polymerization, 90 parts of vinylacetate were added dropwise at a constant speed over 22 hours. When thepolymerization rate of vinyl acetate was 95%, a predetermined amount ofhydroquinone monomethyl ether was added to complete the polymerization,and then distillation was performed while blowing methanol vapor inorder to remove unreacted vinyl acetate monomer out of the system andobtain a methanol solution of vinyl acetate polymer.

Then, the solution was diluted with methanol, the solid contentconcentration was adjusted to 55%, and the methanol solution was addedto a kneader. And saponification was performed by adding a methanolsolution of 2% sodium in sodium hydroxide at a ratio of 6.3 mmol withrespect to 1 mol of vinyl acetate structural unit while maintaining thesolution temperature at 35° C. When the saponification was proceeded,the saponified product was precipitated, and particles were formed,saponification was performed by further adding a methanol solution of 2%sodium in sodium hydroxide at a ratio of 6.0 mmol with respect to 1 molof vinyl acetate structural unit. Then, the acetic acid forneutralization was added at 0.8 equivalent of sodium hydroxide, and PVA1-1 was obtained by filtering, washing well with methanol, and drying ina hot air drier.

The degree of saponification of the obtained PVA 1-1 was 99 mol % byanalyzing the amount of alkali consumption required for hydrolysis ofthe structural unit of the remaining vinyl acetate in the resin.

The average degree of polymerization of PVA 1-1 was 300, based onanalysis according to JIS K 6726.

[Production of PVA 1-1 Pellet]

The PVA 1-1 obtained above was pelletized under the followingconditions.

Extruder: manufactured by Technovel Corporation, 15 mmφ, L/D=60Rotational speed: 200 rpmDischarge amount: 1.2 kg/h to 1.5 kg/hExtrusion temperature:C1/C2/C3/C4/C5/C6/C7/C8/D=90/170/210/220/230/230/230/230/230° C.

The obtained PVA-based resin pellets were evaluated as follows.

[Degree of Crystallinity Measurement]

Five (5) mg of sample of the PVA-based resin pellets obtained above wassealed in a measurement pan, using a heat flux differential scanningcalorimeter “DSC3” manufactured by Mettler Toledo, when the temperaturewas raised from −30° C. to 215° C. at a temperature raising rate of 10°C./min, immediately thereafter, the temperature was lowered to −30° C.at a temperature decreasing rate of 10° C./min, and raised to 230° C. ata temperature raising rate of 10° C./rain again, the heat of fusion (ΔH)of the melting point was calculated to be 69.0 J/g, and the degree ofcrystallinity was 44.0%.

[Dissolvability Evaluation (40° C.)]

Four (4) g of the PVA-based resin pellet obtained above was put into 96g of water at 40° C. and was stirred for 180 minutes while maintainingthe temperature of water at 40° C.

After 180 minutes, the residue of the PVA-based resin pellet thatremained undissolved was filtered, and the concentration of the aqueoussolution excluding the residue was measured. The concentration of theaqueous solution is calculated from the following formula by weighingand collecting an appropriate amount of the PVA-based resin aqueoussolution, putting the solution in a drier at 105° C. and drying for 3hours, cooling to room temperature, and then measuring the mass of thedried residue.

The concentration (mass %) of the aqueous solution=mass (parts) of driedresidue/mass (parts) of the weighted and collected aqueous solution ofPVA-based resin×100

The amount of residue was calculated from the concentration of theaqueous solution and the charged amount of the PVA-based resin pellet,and the dissolution rate was determined. The results are shown in Table1-1.

[No. 1-2]

Evaluation was performed in the same manner as No. 1-1 except that PVA1-1 in No. 1-1 was replaced with PVA 1-2 (a degree of saponification of99 mol %, an average degree of polymerization of 500, an unmodifiedPVA). The results are shown in Table 1-4.

[No. 1-3]

[Production of PVA 1-3]

Ten (10) parts of vinyl acetate (10% of the total was used for initialcharge), 45 parts of methanol, and 0.20 parts of 3,4-diacetoxy-1-butene(10% of the total was used for initial charge) were added to a reactioncan equipped with a reflux condenser, a dropping device, and a stirrer,the temperature was raised under a nitrogen stream while stirring. Afterreaching the boiling point, 0.100 parts of acetyl peroxide were charged,and polymerization was started.

After 0.5 hours from the start of polymerization, 90 parts of vinylacetate and 1.80 parts of 3,4-diacetoxy-1-butene were added dropwise atconstant speed over 22.5 hours. When the polymerization rate of vinylacetate was 95%, a predetermined amount of hydroquinone monomethyl etherwas added to complete the polymerization, and then distillation wasperformed while blowing methanol vapor in order to remove unreactedvinyl acetate monomer out of the system and obtain a methanol solutionof the polymer.

The above solution was then diluted with methanol, the solidsconcentration was adjusted to 55%, the methanol solution was chargedinto a kneader, and saponification was performed by adding a methanolsolution of 2% sodium in sodium hydroxide at a ratio of 6.3 mmol withrespect to a total amount of 1 mol of vinyl acetate structural units and3,4-diacetoxy-1-butene structural units in the copolymer whilemaintaining the solution temperature at 35° C. When the saponificationwas proceeded, the saponified product was precipitated and particleswere formed, saponification was performed by further adding a methanolsolution of 2% sodium in sodium hydroxide at a ratio of 6.0 mmol withrespect to 1 mol of the total amount of the vinyl acetate structuralunits and the 3,4-diacetoxy-1-butene structural units. Then, the aceticacid for neutralization was added at 0.8 equivalent of sodium hydroxide,PVA 1-3 having a 1,2-diol structure in the side chain was obtained byfiltering, washing with methanol, drying in a hot air drier.

The degree of saponification of the obtained PVA 1-3 having a 1,2-dialstructure in the side chain was 99 mol % by analyzing the amount ofalkali consumption required for hydrolysis of the structural unit of theremaining vinyl acetate and 3,4-diacetoxy-1-butene in the resin.

The average degree of polymerization of PVA 1-3 was 300, based onanalysis according to JIS K 6726.

The content of the 1,2-diol structural unit represented by the aboveformula (6) was 1.0 mol % as calculated from the integrated valuemeasured by ¹H-NMR (300 MHz proton NMR, d₆-DMSO solution, internalstandard substance; tetramethylsilane, 50° C.).

[Production of PVA 1-3 Pellet]

The PVA. 1-3 obtained above was pelletized under the followingconditions.

Extruder: manufactured by Technovel Corporation, 15 mmφ, L/D=60Rotational speed: 200 rpmDischarge amount: 1.2 kg/h to 1.5 kg/hExtrusion temperature:C1/C2/C3/C4/C5/C6/C7/C8/D=90/160/200/225/230/230/230/230/230° C.

The obtained PVA-based resin pellets were evaluated as in No. 1-1.

[No. 1-4]

Evaluation was performed in the same manner as No. 1-3 except that PVA1-3 in No. 1-3 was replaced by PVA 1-4 (a degree of saponification of 99mol %, an average degree of polymerization of 600, a content of the1,2-diol structure of 1.0 mol %).

[No. 1-5]

Evaluation was performed in the same manner as No. 1-3 except that PVA1-3 in No. 1-3 was replaced by PVA 1-5 (a degree of saponification of 99mol %, an average degree of polymerization of 450, a content of the1,2-diol structure of 1.0 mol %).

[No. 1-6]

Evaluation was performed in the same manner as No. 1-3 except that PVA1-3 in No. 1-3 was replaced by PVA 1-6 (a degree of saponification of 99mol %, an average degree of polymerization of 500, a content of ethylenegroup of 7.0 mol %).

[No. 1-7]

Evaluation was performed in the same manner as No. 1-3 except that PVA1-3 in No. 1-3 was replaced by PVA 1-7 (a degree of saponification of 99mol %, an average degree of polymerization of 600, a content of the1,2-diol structure of 1.5 mol %).

[No. 1-8]

Evaluation was performed in the same manner as No. 1-3 except that PVA1-3 in No. 1-3 was replaced by PVA 1-8 (a degree of saponification of 99mol %, an average degree of polymerization of 520, a content of the1,2-diol structure of 2.0%) mol %).

[No. 1-9]

Evaluation was performed in the same manner as No. 1-3 except that PVA1-3 in No. 1-3 was replaced by PVA 1-9 (a degree of saponification of 99mol %, an average degree of polymerization of 600, a content of the1,2-diol structure of 3.0 mol %).

[No. 1-10]

Evaluation was performed in the same manner as No. 1-3 except that PVA1-3 in No. 1-3 was replaced by PVA 1-10 (a degree of saponification of99 mol %, an average degree of polymerization of 470, a content of the1,2-diol structure of 3.0 mol %).

[No. 1-11]

Evaluation was performed in the same manner as No. 1-1 except that PVA1-1 in No. 1-1 was replaced by polylactic acid (“ingeo 4032D”manufactured by NatureWorks)

The results of No. 1-1 to 1-11 are shown in Table 1-1.

TABLE 1-1 PVA-base Resin Dissolvability Evaluation Degree of AverageDegree of Modification Degree of Dissolution Rate (mass %)Saponification (mol %) Polymerization Modification Type Rate (mol %)Crystallinity (%) (40° C., 180 minutes) No. 1-1 99 300 Unmodified — 44.01.5 No. 1-2 99 500 Unmodified — 48.8 0.8 No. 1-3 99 300 1,2-diolSide-chain 1.0 41.9 2.5 No. 1-4 99 600 1,2-diol Side-chain 1.0 38.5 2.3No. 1-5 99 450 1,2-diol Side-chain 1.0 30.7 1.5 No. 1-6 99 500 Ethylene7.0 35.7 0.5 No. 1-7 99 600 1,2-diol Side-chain 1.5 31.2 3.2 No. 1-8 99520 1,2-diol Side-chain 2.0 30.6 37.9 No. 1-9 99 600 1,2-diol Side-chain3.0 26.3 96.0 No. 1-10 99 470 1,2-diol Side-chain 3.0 27.9 99.5 No. 1-11Polylactic Acid 0

As described above, since No. 1-1 to 1-10 using the diverting agent ofthe present invention are dissolved in water, they are immediatelyremoved after temporarily sealing fractures and fissures.

That is, an appropriate diverting agent can be selected depending onvarious purposes, such as a case where it is desired to design a longtime to fill the gap or a case where it is desired to design a shorterremoval time after the gap is filled for a certain period of time.

On one hand, since the polylactic acid does not dissolve in water in No.1-11 using the polylactic acid, it is possible to fill the gap for acertain period of time, but it is found that it takes a long time toremove the polylactic acid after the purpose is achieved.

Test Example 2

The production methods of PVA2-1 to PVA2-16 used in Test Example 2 areas follows.

<Production of PVA2-1>

Twenty (20) parts of vinyl acetate (20% of the total was used forinitial charge), 32.5 parts of methanol, and 0.40 parts of3,4-diacetoxy-1-butene (20% of the total was used for initial charge)were added to a reaction can equipped with a reflux condenser, adropping device, and a stirrer, and the temperature was raised under anitrogen stream while stirring. After reaching the boiling point, 0.093parts of acetyl peroxide were charged, and polymerization was started.

After 0.4 hours from the start of polymerization, 80 parts of vinylacetate and 1.6 parts of 3,4-diacetoxy-1-butene were dropped at constantspeed over 11 hours. When the polymerization rate of vinyl acetate was91%, a predetermined amount of m-dinitrobenzene was added to completethe polymerization, and then distillation was performed while blowingmethanol vapor to remove unreacted vinyl acetate monomer out of thesystem to obtain a methanol solution of the polymer.

The above solution was then diluted with methanol, the solidsconcentration was adjusted to 50%, the methanol solution was chargedinto a kneader, and saponification was performed by adding a methanolsolution of 2% sodium in sodium hydroxide at a ratio of 4.8 mmol withrespect to a total amount of 1 mol of vinyl acetate structural units and3,4-diacetoxy-1-butene structural units in the copolymer whilemaintaining the solution temperature at 35° C. When the saponifiedproduct was precipitated and became particulate as the saponificationwas proceeded, 7.5 mmol of a methanol solution of 2% sodium in sodiumhydroxide was further added to the total amount of 1 mol of the vinylacetate structural units and the 3,4-diacetoxy-1-butene structuralunits, and the saponification was performed. Then, 0.8 equivalent ofsodium hydroxide was added to the acetic acid for neutralization,filtered off, washed with methanol, dried in a hot air drier, and amodified PVA-based resin (PVA2-1) having a 1,2-diol structural unit inthe side chain was obtained.

(Degree of Saponification)

The degree of saponification of the PVA2-1 was 99 mol % when analyzedusing the amount of alkali consumption required for hydrolysis of thestructural unit of the remaining vinyl acetate and3,4-diacetoxy-1-butene in the resin according to JIS K 6726.

(Average Degree of Polymerization)

The average degree of polymerization of PVA2-1 was 450 when analyzedaccording to JIS K 6726.

(Modification Rate)

The content rate (modification rate) of the 1,2-diol structural unitrepresented by the formula (6) in PVA2-1 was 1.0 mol % as calculatedfrom the integrated value measured by ¹H-NMR (300 MHz proton NMR,d₆-DMSO solution, internal standard substance; tetramethylsilane, 50°C.).

(Average Particle Diameter)

The average particle diameter of the PVA2-1 was 270 μm as measured by alaser diffraction type particle size distribution analyzer “Mastersizer3000” (manufactured by Spectris Co., Ltd.).

<Production of PVA2-2>

A modified PVA-based resin containing 1,2-diol structural unit in theside chain was obtained in the same manner as the production of PVA2-1,except that 100 parts of vinyl acetate, 23 parts of methanol, and 6parts of 3,4-diacetoxy-1-butene were charged in an initial batch, andthe polymerization was terminated at a polymerization rate of 70%. Thesieved product obtained by sieving the modified PVA-based resin with a.300 μm sieve was named as PVA2-2.

The degree of saponification, the average degree of polymerization, themodification rate, and the average particle diameter were determined inthe same manner as in the PVA2-1. The degree of saponification was 99mol %, the average degree of polymerization was 1200, the modificationrate was 3.0 mol %, and the average particle diameter was 415 μm.

<Production of PVA2-3>

A modified PVA-based resin containing 1,2-diol structural unit in theside chain was obtained in the same manner as the production of PVA2-1,except that 100 parts of vinyl acetate, 23 parts of methanol, and 2parts of 3,4-diacetoxy-1-butene were charged in an initial batch, andthe polymerization was terminated at a polymerization rate of 58%. Thesieved product obtained by sieving the modified PVA-based resin with a300 μm sieve was named as PVA2-3.

The degree of saponification, the average degree of polymerization, themodification rate, and the average particle diameter were determined inthe same manner as the PVA2-1. The degree of saponification was 99 mol%, the average degree of polymerization was 1800, the modification ratewas 1.0 mol %, and the average particle diameter was 410 μm.

<Production of PVA2-4>

15.0 parts of polyoxyethylene monoallylether having oxyethylene grouphaving an average chain length (n) of 15, 85 parts of vinyl acetate, and10.0 parts of methanol were charged into a polymerization can, wereheated up to a reflux state, and then were refluxed for 30 minutes Afterthat, 0.08 mol % of azobisisobutyronitrile was added with respect to theamount of vinyl acetate, and polymerization was started. At each of 2and 4 hours after the start of the reaction, azobisisobutyronitrile wasadded in an amount of 0.08 mol % with respect to the amount of the vinylacetate.

Then, 0.2 parts of m-dinitrobenzene as an inhibitor was added to 20parts of methanol for cooling at approximately 10 hours after the startof the polymerization reaction, and the reaction can jacket was cooledto stop the polymerization reaction to obtain a polyoxyethylenegroup-containing vinyl acetate polymer. The polymerization rate of thepolymer was approximately 95%.

Subsequently, the residual monomer was removed from the solution of thepolyoxyethylene group-containing vinyl acetate polymer obtained above,adjusted to a concentration of 40% by diluting with methanol and chargedinto a kneader, and then saponification was performed by adding amethanol solution of 2% sodium hydroxide in an amount of 3.5 mmol withrespect to 1 mol of vinyl acetate in the copolymer while maintaining thesolution temperature at 35° C. The saponified product was precipitatedand became particulate as the saponification was proceeded. The producedresin was filtered off, washed well with methanol, and dried in a hotair drier to obtain an oxyethylene group-containing PVA-based resin(PVA2-4).

The degree of saponification, the average degree of polymerization, themodification rate, and the average particle diameter were determined inthe same manner as in the PVA2-1. The degree of saponification was 99mol %, the average degree of polymerization was 750, the modificationrate was 2.0 mol %, and the average particle diameter was 287 μm.

<Production of PVA2-5>

Unmodified PVA with a degree of saponification of 73 mol % and anaverage degree of polymerization of 500 was subjected to a heattreatment at 140° C. for 2 hours in a constant-temperature dryer toobtain PVA2-5. The average particle diameter of the obtained PVA2-5 was240 μm.

<Production of PVA2-6>

The PVA2-1 was subjected to a heat treatment at 140° C. for 2 hours in aconstant-temperature dryer to obtain PVA2-6.

The degree of saponification, the average degree of polymerization, themodification rate, and the average particle diameter were determined inthe same manner as in the PVA2-1. The degree of saponification was 99mol %, the average degree of polymerization was 450, the modificationrate was 1.0 mol %, and the average particle diameter was 270 μm.

<Production of PVA2-7>

A rolling flow coating apparatus (MP-01 manufactured by PowrexCorporation) was used, 700 parts of finely pulverized particles (degreeof saponification degree of 88 mol %, average degree of polymerizationof 500, average particle diameter of 100 μm) of PVA2-8 described belowas a core part and 700 parts of a 3% aqueous solution of PVA2-15 (degreeof saponification of 99 mol %, average degree of polymerization of 500,average particle diameter of 258 mi) described later (resin content, 21parts) as a shell part are coated under the following conditions. Thus,core-shell particles of PVA2-7 were obtained.

The average particle diameter was determined in the same manner asPVA2-1. The average particle diameter was 183 μm.

Coating Conditions

Time: 100 minutes

Air supply temperature: 80° C.

Exhaust temperature: 40° C.

Spray speed: 4.5 g/min (40 min) and 7.5 g/min (60 min)

Rotor rotation speed: 300 rpm/min

<Production of PVA2-8>

A modified PVA-based resin containing a 1,2-diol structural unit in aside chain was obtained as PVA2-8.

The degree of saponification, the average degree of polymerization, themodification rate, and the average particle diameter were determined inthe same manner as the PVA2-1. The degree of saponification was 99 mol%, the average degree of polymerization was 600, the modification ratewas 1.5 mol %, and the average particle diameter was 594 μm.

<Production of PVA2-9>

A modified PVA-based resin containing an acetoacetyl group was obtainedas PVA2-9.

The degree of saponification, the average degree of polymerization, themodification rate, and the average particle diameter were determined inthe same manner as the PVA2-1. The degree of saponification degree was99 mol %, the average degree of polymerization was 1100, themodification rate was 5.5 mol %, and the average particle diameter was245 μm.

<Production of PVA2-10>

A carboxylic acid-modified PVA-based resin was obtained as PVA2-10.

The degree of saponification, the average degree of polymerization, themodification rate, and the average particle diameter were determined inthe same manner as the PVA2-1. The degree of saponification degree was99 mol %, the average degree of polymerization was 1700, themodification rate was 2.0 mol %, and the average particle diameter was1100 μm.

<Production of PVA2-11>

A modified PVA-based resin containing a 1,2-diol structural unit in aside chain was obtained as PVA2-11.

The degree of saponification, the average degree of polymerization, themodification rate, and the average particle diameter were determined inthe same manner as the PVA2-1. The degree of saponification was 99 mol%, the average degree of polymerization was 1200, the modification ratewas 1.0 mol %, and the average particle diameter was 215 μm.

<Production of PVA2-12>

A modified PVA-based resin containing a 1,2-diol structural unit in aside chain was obtained as PVA2-12.

The degree of saponification, the average degree of polymerization, themodification rate, and the average particle diameter were determined inthe same manner as the PVA2-1. The degree of saponification degree was92 mol %, the average degree of polymerization was 2500, themodification rate was 2.0 mol %, and the average particle diameter was600 μm.

<Production of PVA2-13>

An unmodified PVA having a degree of saponification of 88 mol % and anaverage degree of polymerization of 500 was produced as PVA2-13.

The average particle diameter was determined in the same manner asPVA2-1. The average particle diameter was 300 μm.

<Production of PVA2-14>

An unmodified PVA having a degree of saponification of 73 mol % and anaverage degree of polymerization of 500 was produced as PVA2-14.

The average particle diameter was determined in the same manner asPVA2-1. The average particle diameter was 240 μm.

<Production of PVA2-15>

An unmodified PVA having a degree of saponification of 99 mol % and anaverage degree of polymerization of 500 was produced as PVA2-15.

The average particle diameter was determined in the same manner asPVA2-1. The average particle diameter was 258 μm.

<Production of PVA2-16>

An unmodified PVA having a degree of saponification of 99 mol % and anaverage degree of polymerization of 1800 was produced as PVA2-16.

The average particle diameter was determined in the same manner asPVA2-1. The average particle diameter was 259 μm.

[No. 2-1]

(Dissolution Rate after 1 Hour)

A 140 mL glass container with a lid containing 100 g of water was placedin a thermostatic chamber, and the water temperature was set to 40° C.The long sides of 120 mesh (aperture 125 μm, 10 cm×7 cm) made of nylonwere folded in half, and both ends were heat-sealed to obtain a mesh bag(5 cm×7 cm).

One (1) g of the PVA2-1 was put into the obtained mesh bag, and theopening was heat-sealed. Then, a mesh bag containing the PVA2-1 wasobtained, and the mass was measured. The mesh bag containing the PVA2-1was immersed in the glass container. The mesh bag containing the PVA2-1was taken out from the glass container after standing for 1 hour at athermostatic chamber at 40° C. and then dried at 105° C. for 3 hours.The mass of the mesh bag containing the PVA2-1 was measured, the mass ofthe PVA2-1 remaining in the mesh bag was calculated from the mass beforeimmersion, and the dissolution rate after 1 hour of the PVA2-1 wascalculated by the following formula (Y). The dissolution rate after 1hour was 25 mass %.

[Equation6] $\begin{matrix}{\begin{matrix}{{Dissolution}{rate}} \\{{after}1{hour}} \\{{of}{the}{immersion}} \\\left( {{mass}\%} \right)\end{matrix} = {\left\{ {\begin{matrix}{{1(g)} -} \\{1{(g) \times}}\end{matrix}\frac{\begin{matrix}{{{M{ass}}(g){of}{the}{polyvinyl}{alcohol}}‐} \\{{based}{resin}{remaining}{in}{the}{mesh}{bag}}\end{matrix}}{\frac{\begin{matrix}{{Solid}{fraction}\left( {{mass}\%} \right){of}{polyvinyl}} \\{{alcohol}‐{{based}{resin}}}\end{matrix}}{100}}} \right\} \times 100}} & (Y)\end{matrix}$

(Dissolution Rate after 24 Hours)

The dissolution rate after 24 hours of PVA 2-1 can be calculated bycalculating the mass of the PVA2-1 remaining in the mesh bag after 24hours in the same manner except that standing for 1 hour in the step ofcalculating the dissolution rate after 1 hour is changed to standing for24 hours. The dissolution rate after 24 hours was 74 mass %.

In the PVA2-1, the ratio of the dissolution rate after 24 hours withrespect to the dissolution rate after 1 hour determined by thedissolution rate (mass %) after 24 hours/the dissolution rate (mass %)after 1 hour was 3.0.

The dissolution rate (mass %) after 1 hour of PVA2-1, the dissolutionrate (mass %) after 24 hours (mass %), and a ratio of the dissolutionrate after 24 hours with respect to the dissolution rate after 1 hourwere collected in Table 2-1.

[No. 2-2 to No. 2-16]

The same test as in No. 2-1 was performed. using PVA2-2 to PVA2-16instead of PVA2-1 in No. 2-1. The results are shown in Tables 2-1 to2-2.

TABLE 2-1 Dissolution Rate after 24 Hours (mass %)/ Average AverageDissolution Dissolution Dissolution Degree of Degree of ModificationParticle Rate after 1 Rate after Rate after Type of SaponificationPolymeriza- Modification Rate Diameter Heat Hour 24 Hours 1 Hour PVA(mol %) tion Type (mol %) (μm) Treatment (mass %) (mass %) (mass %) No.2-1 PVA2-1 99 450 1,2-diol 1.0 270 NULL 25 74 3.0 Side-chain No. 2-2PVA2-2 99 1200 1,2-diol 3.0 415 NULL 10 68 6.8 Side-chain No. 2-3 PVA2-399 1800 1,2-diol 1.0 410 NULL 6 33 5.5 Side-chain No. 2-4 PVA2-4 99 750Oxyethylene 2.0 287 NULL 16 94 5.9 Group No. 2-5 PVA2-5 73 500Unmodified — 240 140° C. 20 99 5.0 2 Hours No. 2-6 PVA2-6 99 4501,2-diol 1.0 270 140° C. 9 69 7.7 Side-chain 2 Hours No. 2-7 PVA2-7 CorePart 88 Core Part 500 Unmodified — 183 NULL 27 95 3.5 Shell Part 99Shell Part 500

TABLE 2-2 Dissolution Rate after 24 Hours (mass %)/ Average AverageDissolution Dissolution Dissolution Degree of Degree of ModificationParticle Rate after 1 Rate after Rate after Type of SaponificationPolymeriza- Modification Rate Diameter Heat Hour 24 Hours 1 Hour PVA(mol %) tion Type (mol %) (μm) Treatment (mass %) (mass %) (mass %) No.2-8 PVA2-8 99 600 1,2-diol 1.5 594 NULL 14 5 5.0 Side-chain No. 2-9PVA2-9 99 1100 Acetoacetyl 5.5 245 NULL 5 54 10.8 Group No. 2-10 PVA2-1099 1700 Carboxylic 2.0 1100 NULL 7 99 14.1 Acid No. 2-11 PVA2-11 99 12001,2-diol 1.0 215 NULL 0.1 34 340 Side-chain No. 2-12 PVA2-12 92 25001,2-diol 2.0 600 NULL 7 99 14.1 Side-chain No. 2-13 PVA2-13 88 500Unmodified — 300 NULL 35 91 2.6 No. 2-14 PVA2-14 73 500 Unmodified — 240NULL 36 98 2.7 No. 2-15 PVA2-15 99 500 Unmodified — 258 NULL 7 15 2.1No. 2-16 PVA2-16 99 1800 Unmodified — 259 NULL 9 14 1.6

The diverting agent of No. 2-1 to No. 2-12 has the further suppressedinitial dissolution rate after 1 hour and the more excellent dissolutionrate after 24 hours in water.

A diverting agent containing a PVA-based resin of which a ratio of adissolution rate of 24 hours with respect to a dissolution rate after 1hour is 2.8 or more tends to be able to maintain the shape for a certainperiod of time after being added to water, so that the diverting agentis easy to temporarily fill fractures formed in the shale layer andeasily dissolved in water when petroleum or natural gas or the like iscollected.

Test Example 3

The production methods of PVA3-1 to PVA3-6 used in Test Example 3 are asfollows.

<Production of PVA3-1>

One hundred (100) parts of vinyl acetate, 23 parts of methanol, and 2parts of 3,4-diacetoxy-1-butene were added to a reaction can equippedwith a reflux condenser, a dropping device, and a stirrer, thetemperature was raised under a nitrogen stream while stirring. Afterreaching the boiling point, 0.014 parts of acetyl peroxide were charged,and polymerization was started.

When the polymerization rate of vinyl acetate was 58%, a predeterminedamount of m-dinitrobenzene was added to complete the polymerization, andthen distillation was performed while blowing methanol vapor to removeunreacted vinyl acetate monomer out of the system to obtain a methanolsolution of the polymer.

Then, the above solution was then diluted with methanol, the solidsconcentration was adjusted to 50%, the methanol solution was chargedinto a kneader, and saponification was performed by adding a methanolsolution of 2% sodium in sodium hydroxide at a ratio of 4.1 mmol withrespect to a total amount of 1 mol of vinyl acetate structural units and3,4-diacetoxy-1-butene structural units in the copolymer whilemaintaining the solution temperature at 35° C. When the saponifiedproduct was precipitated and became particulate as the saponificationwas proceeded, 7.5 mmol of a methanol solution of 2% sodium in sodiumhydroxide was further added with respect to the total amount of 1 mol ofthe vinyl acetate structural units and the 3,4-diacetoxy-1-butenestructural units, and the saponification was performed. Then, the aceticacid for neutralization was added at 1.0 equivalent of sodium hydroxide,and a modified PVA-based resin having a 1,2-diol structural unit in theside chain was obtained by filtering, washing with methanol, drying in ahot air drier. The sieved product obtained by sieving the modifiedPVA-based resin with a 300 μm sieve was named as PVA3-1.

(Degree of Saponification)

The degree of saponification of the PVA3-1 was 99 mol % when analyzedusing the amount of alkali consumption required for hydrolysis of thestructural unit of the remaining vinyl acetate and3,4-diacetoxy-1-butene in the resin according to JIS K 6726.

(Average Degree of Polymerization)

The average degree of polymerization of PVA3-1 was 1800 when analyzedaccording to JIS K 6726.

(Modification Rate)

The content rate (modification rate) of the 1,2-diol structural unitrepresented by the formula (1) in PVA3-1 was 1 mol % as calculated fromthe integrated value measured by ¹H-NMR (300 MHz proton NMR, d₆-DMSOsolution, internal standard substance; tetramethylsilane, 50° C.).

(Average Particle Diameter)

The average particle diameter of the PVA3-1 was 410 μm as measured by alaser diffraction type particle size distribution analyzer “Mastersizer3000” (manufactured by Spectris Co. Ltd.).

(Degree of Swelling and Elution Rate)

A 140 mL glass container with a lid was charged with 100 g ofion-exchanged water, and 1 g of PVA3-1 was added to prepare a PVA3-1aqueous solution, and then was left to stand for 1 day in a thermostaticchamber at 23° C. Then, the PVA3-1 aqueous solution was filtered througha 120 mesh (opening 125 μm) made of nylon, and the mass of PVA3-1(PVA3-1 after swelling) remaining on the sieve was measured. Next, thePVA3-1 after swelling was dried at 140° C. for 3 hours, and the mass ofPVA3-1 after drying was measured. The degree of swelling of PVA3-1 was16 by determining by the following formula (B).

[Equation7] $\begin{matrix}{{{Degree}{of}{swelling}} = \frac{\begin{matrix}{{{Mass}(g){of}{polyvinyl}{alcohol}}‐{{{based}{resin}{after}{swelling}} -}} \\{{{mass}(g){of}{polyvinyl}{alcohol}}‐{{based}{resin}{dried}{after}{swelling}}}\end{matrix}}{{{mass}(g){of}{polyvinyl}{alcohol}}‐{{based}{resin}{dried}{after}{swelling}}}} & (B)\end{matrix}$

The elution rate (mass %) of PVA3-1 was 29 mass % by determining by thefollowing formula (C).

In the following formula (C), the solid fraction (mass %) of thepolyvinyl alcohol-based resin can be calculated by drying the PVA-basedresin at 105° C. for 3 hours and measuring the mass of the PVA-basedresin before and after drying.

[Equation8] $\begin{matrix}{{{Elution}{rate}\left( {{mass}\%} \right)} = {\left\{ {\begin{matrix}{{1(g)} -} \\{1{(g) \times}}\end{matrix}\frac{{{M{ass}}(g){of}{polyvinyl}{alcohol}}‐{{based}{resin}{dried}{after}{swelling}}}{\frac{\begin{matrix}{{Solid}{fraction}\left( {{mass}\%} \right){of}{polyvinyl}} \\{{alcohol}‐{{based}{resin}}}\end{matrix}}{100}}} \right\} \times 100}} & (C)\end{matrix}$

Of PVA3-1, the value of the degree of swelling×elution rate was 464.

<Production of PVA3-2>

An unmodified PVA having a degree of saponification of 99 mol % and anaverage degree of polymerization of 1800 was produced as PVA3-2.

The average particle diameter, the degree of swelling, and the elutionrate were determined in the same manner as PVA3-1. The average particlediameter was 259 μM, the degree of swelling was 3, the elution rate was8 mass %, and the value of the degree of swelling×elution rate was 24.

<Production of PVA3-3>

An unmodified PVA having a degree of saponification of 99 mol % and anaverage degree of polymerization of 500 was produced as PVA3-3.

The average particle diameter, the degree of swelling, and the elutionrate were determined in the same manner as PVA3-1. The average particlediameter was 258 μm, the degree of swelling was 2, the elution rate was8 mass %, and the value of the degree of swelling×elution rate was 16.

<Production of PVA3-4>

A modified PVA-based resin containing 1,2-diol structural unit in theside chain was obtained in the same manner as in the production ofPVA3-1, except that 100 parts of vinyl acetate, 23 parts of methanol,and 2 parts of 3,4-diacetoxy-1-butene were charged in an initial batch,and the polymerization was terminated at a polymerization rate of 70%.The modified PVA-based resin was put in a constant-temperature dryer setto 140° C., and was subjected to a heat treatment for 2 hours to obtaina modified PVA-based resin as PVA3-4.

The degree of saponification, the average degree of polymerization, themodification rate, the average particle diameter, the degree ofswelling, and the elution rate were determined in the same manner as thePVA3-1. The degree of saponification degree was 99 mol %, the averagedegree of polymerization was 1100, the modification rate was 1 mol %,the average particle diameter was 400 μm, the degree of swelling was 8,the elution rate is 20 mass %, and the degree of swelling×elution ratewas 160.

<Production of PVA3-5>

A modified PVA resin containing an ethylene group having a degree ofsaponification of 99 mol % and an average degree of polymerization of500 was used as PVA3-5.

The average particle diameter, the degree of swelling, and the elutionrate were determined in the same manner as PVA3-1. The average particlediameter was 700 μm, the degree of swelling was 6, the elution rate was2 mass %, and the value of the degree of swelling×elution rate was 12.

<Production of PVA3-6>

An unmodified PVA having a degree of saponification of 88 mol % and anaverage degree of polymerization of 500 was produced as PVA3-6.

The average particle diameter, the degree of swelling, and the elutionrate were determined in the same manner as PVA3-1. The average particlediameter was 300 μm, the degree of swelling was 11, the elution rate was82 mass %, and the value of the degree of swelling×elution rate was 902.

[No. 3-1]

A 140 mL glass container with a lid was charged with 100 g of ionexchanged water and rotator, and as stirred at 750 rpm at roomtemperature. While stirring, 6 g of PVA 3-1 was charged, and stirringwas continued for 1 minute to obtain an aqueous dispersion of PVA3-1.After that, stirring was stopped and the aqueous dispersion of PVA3-1was allowed to stand for 5 minutes, and the aqueous dispersion of PVA3-1was stirred again at 750 rpm for 30 seconds, and the state of theaqueous dispersion of PVA3-1 at that time was visually observed andevaluated based on the following criteria. The results are shown inTable 3-1.

A: The PVA-based resin was uniformly dispersed in an aqueous solution.

B: Immediately after stirring, the PVA-based resin was dispersed in anaqueous solution. When the stirring was continued, the PVA-based resinparticles swelled and was difficult to be stirred.

C: The PVA-based resin was adhered to each other, and the PVA-basedresin was not uniformly dispersed in the aqueous solution.

[No. 3-2 to No. 3-6]

The same test as in No. 3-1 was performed using PVA3-2 to PVA3-6 insteadof PVA3-1 in No. 3-1. The results are shown in Table 3-1.

TABLE 3-1 Average Average Degree of Degree of Degree of ModificationParticle Elution Swelling × Type of Saponification Polymeriza-Modification Rate Heat Diameter Degree of Rate Elution Dispersion PVA(mol %) tion Type (mol %) Treatment (μm) Swelling (mass %) Rate Test No.3-1 PVA3-1 99 1800 1,2-diol 1 NULL 410 16 29 464 B Side-chain No. 3-2PVA3-2 99 1800 Unmodified — NULL 259 3 8 24 A No. 3-3 PVA3-3 99 500Unmodified — NULL 258 2 8 16 A No. 3-4 PVA3-4 99 1100 1,2-diol 1 140° C.400 8 20 160 A Side-chain 2 Hours No. 3-5 PVA3-5 99 500 Ethylene 7 NULL700 6 2 12 A No. 3-6 PVA3-6 88 500 Unmodified — NULL 300 11 82 902 C

From the results in Table 3-1, it was found that No. 3-1 to No. 3-5using a PVA-based resin of which a value of the degree ofswelling×elution rate is 500 or less had better dispersibility of thePVA-based resin in the aqueous solution than that of No. 3-6.

It was found that No. 3-1 to No. 3-5 having a higher degree ofsaponification than that of No. 3-6 had better dispersibility of thePVA-based resin in the aqueous solution.

Furthermore, it was found that the values of the degree ofswelling×elution rate of No. 3-2 and No. 3-3 using an unmodified PVA andNo. 3-5 using an ethylene-modified PVA-based resin were lower among No.3-1 to No. 3-5.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope of the invention. This applicationis based on Japanese Patent Application (Patent Application No.2017-155040) filed on Aug. 10, 2017, Japanese Patent Application (PatentApplication No. 2017-254842) filed on Dec. 28, 2017, and Japanese PatentApplication (Patent Application No. 2017-254843) filed on Dec. 28, 2017,the contents of which are incorporated herein by reference.

1. A diverting agent comprising: a polyvinyl alcohol-based resin,wherein an average degree of polymerization of the polyvinylalcohol-based resin is from 150 to
 4000. 2. The diverting agentaccording to claim 1, wherein the polyvinyl alcohol-based resin has adegree of saponification of 90 mol % or more.
 3. The diverting agentaccording to claim 1, wherein the polyvinyl alcohol-based resin has adissolution rate of 0.1 mass % to 30 mass %, when 4 g of the polyvinylalcohol-based resin is charged into 96 g of water and is stirred for 180minutes at 40° C.
 4. The diverting agent according to claim 1, wherein adegree of crystallinity of the polyvinyl alcohol-based resin is 25% to60%.
 5. The diverting agent according to claim 1, wherein when 1 g ofthe polyvinyl alcohol-based resin is immersed in 100 g of water at 40°C., a ratio of a dissolution rate after 24 hours with respect to adissolution rate after 1 hour of the polyvinyl alcohol-based resin is2.8 or more.
 6. The diverting agent according to claim 5, wherein when 1g of the polyvinyl alcohol-based resin is immersed in 100 g of water at40° C., the dissolution rate after 1 hour is less than 30 mass %.
 7. Thediverting agent according to claim 5, wherein when 1 g of the polyvinylalcohol-based resin is immersed in 100 g of water at 40° C., thedissolution rate after 24 hours is 30 mass % or more.
 8. The divertingagent according to claim 5, wherein the polyvinyl alcohol-based resin isa modified polyvinyl alcohol-based resin.
 9. The diverting agentaccording to claim 8, wherein a modification rate of the modifiedpolyvinyl alcohol-based resin is 0.5 mol % to mol %.
 10. The divertingagent according to claim 1, wherein the polyvinyl alcohol-based resinsatisfies formula (A):degree of swelling×elution rate (mass %)≤500  (A) wherein the degree ofswelling is a value determined according to formula (B): $\begin{matrix}{{{Degree}{of}{swelling}} = \frac{\begin{matrix}{{{Mass}(g){of}{polyvinyl}{alcohol}}‐{{{based}{resin}{after}{swelling}} -}} \\{{{mass}(g){of}{polyvinyl}{alcohol}}‐{{based}{resin}{dried}{after}{swelling}}}\end{matrix}}{{{mass}(g){of}{polyvinyl}{alcohol}}‐{{based}{resin}{dried}{after}{swelling}}}} & (B)\end{matrix}$ wherein a mass in grams of the polyvinyl alcohol-basedresin after swelling is the mass of a residual polyvinyl alcohol-basedresin obtained by charging 1 g of a polyvinyl alcohol-based resin into100 g of water, leaving it to stand for 1 day in a thermostatic chamberat 23° C., collecting a filtrate by filtration, and drying the filtrateat 140° C. for 3 hours, and wherein the elution rate, in mass %, is avalue determined according to formula (C): $\begin{matrix}{{{Elution}{rate}\left( {{mass}\%} \right)} = {\left\{ {\begin{matrix}{{1(g)} -} \\{1{(g) \times}}\end{matrix}\frac{{{M{ass}}(g){of}{polyvinyl}{alcohol}}‐{{based}{resin}{dried}{after}{swelling}}}{\frac{\begin{matrix}{{Solid}{fraction}\left( {{mass}\%} \right){of}{polyvinyl}} \\{{alcohol}‐{{based}{resin}}}\end{matrix}}{100}}} \right\} \times 100}} & (C)\end{matrix}$ wherein the mass of the polyvinyl alcohol-based resindried after swelling is the same as defined for formula (B).
 11. Thediverting agent according to claim 10, wherein the elution rate of thepolyvinyl alcohol-based resin is 50 mass % or less.
 12. The divertingagent according to claim 10, wherein the degree of swelling of thepolyvinyl alcohol-based resin is 30 or less.
 13. A method of filling afracture which is a method of temporarily filling the fracture generatedin a well, comprising: allowing the diverting agent according to claim 1to flow into a fracture to be filled with a flow of fluid in the well.